An Alaska Airlines jet headed for Seattle had to make an emergency landing in Portland, Oregon on Thursday night after its pilot lost consciousness.

The co-pilot of the Boeing 737-700 was able to take control of the airplane, while a doctor on board tended to the pilot. The co-pilot, an 11-year veteran of Alaska Airlines, landed the plane at Portland International Airport, where emergency medics were waiting on the runway. The pilot, who has not yet been identified, was taken to a local hospital, but there was no immediate word on his condition. It wasn't immediately clear if the crew alerted passengers to the in-flight incident.

Flight 473 had been scheduled to arrive in Seattle at 9:30 p.m., but landed in Portland at around 9:05 p.m. About 20 of the 116 passengers on board  with connecting flights were flown to Seattle on a shuttle. The other passengers remained onboard the jet until a new pilot was located, multiple news reports said. No one was injured.

In an interview with ABC News, one passenger described what happened.  "I'm seeing the cockpit open completely and the pilot had come out, barged through the door, and fell on the aisle. He either passed out or had a stroke or something," the passenger said. "Then the flight attendant was going up and down the aisle saying, 'Are there any paramedics, doctors, nurses, someone that can come and help?'"

Alaska Airlines spokesman Paul McElroy said it remains unknown what caused the pilot, who has 28 years of flying experience with the carrier, to pass out. McElroy said that the pilot was up-to-date on his six-month medical evaluation.

"Doctors suspect he became ill due to food-poisoning or the flu virus," McElroy said.

The incident marks the second time in a month that an Alaska Airlines pilot has fainted during a flight.

On Jan. 22, a co-pilot lost consciousness between Seattle and Las Vegas, and the pilot of that flight had to make an emergency landing. “That first officer was infected with the flu. We know that for sure," McElroy said, describing that incident as a “bizarre coincidence.”